Method for producing gas diffusion electrodes

ABSTRACT

With the help of a method for production of a gaseous diffusion electrode from a silver catalyst on PTFE-substrate, it is endeavored to achieve results which can be reproduced, by avoiding the disadvantages of the state-of-the-art technology, whereby this is achieved in that
         the porous system of the silver catalyst is filled with a wetted fluid;   a dimension-stable solid body with a grain size above that of the silver catalyst is mixed below the silver catalyst;   the thus compression-stable mass is shaped into a homogenous catalyst band in a calender; and   in a second calender step, an electrically conductive conductor material is imprinted into the catalyst band.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of German Application No.101 30 441.2 filed Jun. 23, 2001. Applicants also claim priority under35 U.S.C. §365 of PCT/EP02/06706 filed Jun. 18, 2002. The internationalapplication under PCT article 21(2) was not published in English.

This invention pertains to a method for manufacturing porous gaseousdiffusion electrodes of the generic type mentioned in claim 1. Such agaseous diffusion electrode can, for example, be based on a catalyticactive silver or silver alloys for use in electro-chemical cells,particularly of choloro-alkaline electrolysis, or alkaline fuel cells.

In electro-chemical cells, the reduction of oxygen is carried out onplatinum, silver or even carbon. Platinum can be used in acidic as wellas in alkaline surroundings, whereas silver and carbon are stableagainst corrosion only in alkaline electrolytes. However, in case ofsilver catalyst, even in alkaline mediums rapid deactivation occurs,which can be explained due to rearrangement of the oxidic surface of thesilver. (Texas Instruments, U.S. Pat. No. 3,505,129). It has been triedseveral times to reduce the corrosive attack of silver alloyingpartners. Thereby one knows of alloys with precious material likeplatinum, paladium, gold and mercury (DE 20 21 009), or even withnon-precious substances like nickel (DE 15 46 728), copper and othermaterials. It has also been attempted to achieve a stabilisation of thesilver by means of refining or also with the help of anodic corrosionprotection (local elements). In case of corrosion, initially a silveroxide surface is formed. As silver oxide is relatively well soluble inlye solutions, a rearrangement of silver crystals can take place. InFIGS. 2 and 3, REM-images of silver electrodes before and afteroperation have been depicted. One can very clearly identify thereduction of the inner porous structure. The catalytic activity getsreduced.

Apart from stabilisation, a method of manufacture of an active silvercatalyst must also ensure that the active surface of the silver issufficiently large, i.e. the grain size of the silver should be as smallas possible. Thus, for example, from the document (U.S. Pat. No.3,668,101) it is known that very active silver catalysts can be attainedwith particle diameters of 5 to 10 μm.

We also know of further methods, in which it has been attempted tomanufacture the smallest particles of stable silver alloys. Adequatelysmall silver particles are generated through precipitation procedures.Apart from controlling the pH-value, the temperature and theover-saturation, so-called crystallisation germs play an excellent rolein manufacturing the smallest silver particles. We know of a method (EP0 115 845), in which a mixture of silver nitrate and mercury nitrate areprecipitated on a PTFE-dispersion by adding potash lye. In this way, asilver amalgam with smallest particle diameter is produced.

In order to produce so called gaseous diffusion electrodes from thesecatalysts, as required in fuel cells or in chloro-alkaline electrolysis,the powder should be processed to a homogeneous, flat electrode. Thiselectrode must be electrically conductive and allow the entry of theelectrolyte as well as the gas. It must be possible to wet some parts ofthe electrode, whereas the other parts must be protected from wetting. Asolution to this problem was presented with the help of a bi-porous porestructure. Initially the electrolyte can penetrate without any troubleinto the small as well as the large pores. By means of an over-pressureof the gas or by gravitational force, the electrolyte is again removedfrom the larger pores. Such bi-porous structures work satisfactorilyonly when there is a pressure difference between the gaseous chamber andthe electrolyte chamber. Whether such pressure differences can becreated, if in the electrolytes a membrane separates the anode and thecathode from one another, is doubtful. Thus, these electrodes can not beused in chloro-alkaline electrolysis or in fuel cells with alkalinemembranes.

Hence it was attempted to create a bi-porous pore system with the helpof material properties. This means one requires hydrophilic andhydrophobic materials. Suitable hydrophobic materials are somethermoplasts—e.g. polytetraflouroethylene. The mentioned catalysts andalso the silver are always hydrophilic. Therefore, if one mixes silverand PTFE together and creates a plane electrode from that, then thiscould have different regions with hydrophilic and hydrophobicproperties. Mostly an additional metal conductor is integrated, in orderto achieve better electrical properties.

We know of a new method of producing a so-called gaseous diffusionelectrode from mixtures of PTFE (Polytetraflouroethylene) and catalyst.Thus for example, in the document (EP 0 115 845) it is suggested thatthe materials be suspended in such a manner, that one obtains a pastewhich can be cast, pressed and dried to a particular shape. Thedisadvantage in case of such “pastisised” electrodes is thenon-homogeneous distribution of the materials—there could easily besmall holes through which then gas or electrolyte can penetrate. Inorder to avoid this, the electrodes are manufactured with at least 0.8mm thickness. In this way, there are also huge silver quantities in theelectrodes (around 2 Kg/m²), so that the price advantage of silver isagain lost. Because, we already know of commercial oxygen electrodeswith approx. 4–40 gm/m² of platinum/carbon.

Two methods are known, in which from such hydrophobic/hydrophilicmaterials a thin, homogeneous gaseous diffusion electrode is rolled.According to the method (EP 0 144 002, U.S. Pat. No. 4,696,872), thecatalyst particles and the PTFE are mixed in a special mixer in such away, that a fine-meshed hydrophobic net system gets precipitated on thecatalyst. The loose mass is then rolled together in a powder roller toform a foil of approx. 0.2 mm thickness. This method has proved usefulfor mixtures with PTFE and carbon, or PTFE and Raney-nickel. It issimilarly possible to in this way roll a Raney-silver-alloy with 80%aluminum into a porous foil. FIG. 1 shows such a calender rollingmechanism. However, it is not possible to process the ductile silver. Incase of the required pressing pressure—approx. 0.01 to 0.6

cm²—in such powder rollers, PTFE and silver are pressed to form acompact, gas-impermeable and electrolyte-impermeable foil. Thecurrent-voltage graph for such an electrode is shown in FIG. 5.

In order to nevertheless be able to produce silver electrodes, initiallya silver oxide/PTFE-mixture is processed in the powder roller andsubsequently reduced electro-chemically (DE 37 10 168). The silver oxideis stable enough to withstand the pressing pressure of the roller.Besides, the volume reduces on transition from silver oxide to silver,so that additional pores are generated in the gaseous diffusionelectrodes. By means of the parameters during reduction, the grain sizeof the particles can be very well adjusted. The disadvantage of thismethod is, that it is not yet known, how silver alloys with catalyticproperties can be reduced electro-chemically. Hence it is not possibleto produce durable, stable silver electrodes by means ofelectro-chemical reduction.

The task of this invention is to present a method for producing agaseous diffusion electrode, with which not only the disadvantages ofthe state-of-the-art technology can be avoided, but also to particularlyevolve results in the process product which can be reproduced.

With the help of the method already mentioned above, this task isfulfilled according to the invention, in that

-   -   The porous system of the silver catalyst is filled with a        wetting fluid;    -   a dimension-stable solid body with a grain size above that of        the silver catalyst mixed below the silver catalyst;    -   the thus formed compression-stable mass is shaped in a calender        to a homogeneous catalyst band; and    -   in a second calender step, an electrically conducting material        is imprinted in the catalyst band.

The speciality of this method as per the invention lies therein, thatthe inner porous system of the ductile material is filled with a fluid.As this fluid cannot be solidified and, on the other hand, is fixed inthe porous system by means of the capillary forces, the fluid cannot beremoved from the micro-pores even at a pressure of maximum 600 kg/cm².Further addition of a little powder carbon or the volatile ammoniumcarbonate can take up the mechanical pressure of the powder roller evenfurther. By means of these coarse-grained additions of typically 10–100μm grain diameter, the porous system with larger pore diameter isprotected from solidification. By means of a subsequent annealing step,the fluid as well as the ammonium carbonate can be driven out of theelectrode. In this way, one can obtain large pores in the gaseousdiffusion electrode, which ensures rapid gas transportation and smallerpores in the catalyst, which allow a homogeneous optimum utilisation ofthe catalyst.

A preferred execution of this method is depicted as follows:

-   first, silver or a silver alloy is produced by means of a    precipitation process. Thereby, it would be advantageous to carry    out the precipitation on a PTFE-dispersion. The best experiences are    made with a mixture of 15% teflon and 85% silver. By addition of    formaldehyde during precipitation, the silver hydroxide immediately    gets transformed in the alkaline surroundings into a silver crystal.    The precipitate mass is washed and dried. Subsequent annealing at    200° C. improves the electrical contact between the silver particles    and drives out the remaining fluids.

A quantity of about 5%-40%, preferably however 8%, of a fluid is addedto this powder. This fluid can penetrate into the porous system of thePTFE and the silver. On account of the hydrophobic character of thePTFE, only isopropanol, ethanol and methanol will come intoconsideration. If the powder is wetted and filled with such solvents,then there could subsequently be an exchange of the fluids. For example,one can bring a powder immersed in isopropanol into a water bath, orglycerine, and thus within hours the fluids get exchanged throughdiffusion. In this way, fluid enters into the porous system of the PTFE,which is generally repelled by the PTFE. The thus moistened materialbehaves externally like a powder because the fluid is present in theinner porous system.

Another generic type of wetting agent would be the so-called tensides.These penetrate into the porous system, and at the same time also coverthe surface of the catalyst, thus reducing its surface roughness. Thisreduced surface roughness leads during the rolling process to thephenomenon, that the silver catalyst can move away from thesolidification zone, whereas other powder components which have not beentreated remain in the solidification zone and thus produce the electrodecombination in which the silver catalyst is embedded (FIG. 4). Such apowder could be ammonium carbonate or activated carbon, which can now bemixed to a homogeneous mass with the silver catalyst in a pulverizer, asdescribed in EP 0 144 002. Subsequently, the loose mass is rolled into afoil of approx. 0.2 mm thickness by means of a powder roller.

In a second pair of rollers, a metallic support structure can be rolledin the form of woven nets or stretch-metals and thus the mechanicalstability and the electrical conductivity can be improved. After thissequence, the gaseous diffusion electrode is dried. Thereafter theelectrode has a silver deposit between 0.2 kg/m² and 1.5 kg/m².Generally, one endeavours for a weight of approx. 0.5 kg/m². Thus, up to75% of the hitherto required silver can be saved. In spite of thereduced silver weight, with such electrodes one obtains acurrent-voltage-graph as shown in FIG. 6.

Of course, this method can also be combined with others. Thus, one cando away with the environmentally harmful formaldehyde for precipitationand instead carry out the reduction after production of the gaseousdiffusion electrodes by means of electro-chemical methods. In this way,one can similarly produce alloys by carrying out a Ko-precipitation ofsilver and mercury, titan, nickel, copper, cobalt or bismuth.

Especially for the chloro-alkaline-electrolysis, changes can be effectedon the ready gaseous diffusion electrode, which would enable improvedremoval of the occurring soda lye. For this, the imprinting of a coarseconducting system is advisable. This is possible, if a net is pressed onto the ready electrode and then subsequently removed again. The negativeimpression of the net forms channels, in which the electrolyte can laterflow off parallel to the electrode surface.

Further features, details and advantages of the invention are shown inthe following diagrams. The following are shown:

FIG. 1 A functional diagram of a device/plant as per the invention;

FIG. 2 A microscopic image of a silver electrode before use;

FIG. 3 In the same depiction form, a silver electrode after use;

FIG. 4 A PTFE-structure embedded in a silver catalyst;

FIG. 5 A current/voltage diagram of a chloro-alkaline electrolysis; and

FIG. 6 The same graph according to the parameters of the invention.

1. Method for producing a gaseous diffusion electrode from a silvercatalyst on a PTFE-substrate, comprising the steps of: filling a poroussystem of the silver catalyst with a wetting fluid; mixing adimension-stable solid body with a grain size above that of the silvercatalyst with the silver catalyst to produce a compression-stable mass;shaping the thus obtained compression-stable mass into a homogeneouscatalyst band in a calender, and in a second calender step, imprintingan electrically conducting material into the catalyst band.
 2. Method asper claim 1, comprising using as wetting fluid 5% isopropanol and assolid substance 30% ammonium carbonate or ammonium-hydrogen-carbonateand driving both these filling substances out after producing theelectrodes by means of an annealing step at preferably 110° C.
 3. Methodas per claim 1, comprising using as wetting fluid a tenside—preferably5% triton X 100—, which penetrates into the porous system of thecatalyst as well as reduces surface friction, so that the silvercatalyst can glide out of a solidification zone and a dimension-stableammonium carbonate and a PTFE-binder takes up a roller pressure. 4.Method as per claim 1, comprising using in the first calender step, ahomogeneous catalyst band of thickness between 0.2–0.5 mm.
 5. Method asper claim 1, comprising adjusting a roller gap to 350 μm and setting aroller feed to approx. 2 meters per minute.
 6. Method as per claim 1,comprising using as electrical conducting material, a silver-coatednickel wire net with a string thickness of 0.15 mm and mesh width of0.45 mm with an approx. 10 μm thick silver precipitate.
 7. Method forproducing a gaseous diffusion electrode from a silver catalyst on aPTFE-substrate, comprising the steps of: filling a porous system of thesilver catalyst with a wetting fluid; mixing a dimension-stable solidbody with a grain size above that of the silver catalyst with the silvercatalyst to produce a compression-stable mass; shaping the thus obtainedcompression-stable mass into a homogenous catalyst band in a calender,in a second calender step, imprinting an electrically conductingmaterial into the catalyst band; and obtaining said gaseous diffusionelectrode having large pores in the gaseous diffusion electrode, whichensures rapid gas transportation and smaller pores in the catalyst,which allow a homogeneous optimum utilisation of the catalyst.
 8. Methodas per claim 7, comprising using as wetting fluid 5% isopropanol and assolid substance 30% ammonium carbonate or ammonium-hydrogen-carbonateand driving both these filling substances out after producing theelectrodes by means of an annealing step at preferably 110° C.
 9. Methodas per claim 7, comprising using as wetting fluid a tenside—preferably5% triton X 100—, which penetrates into the porous system of thecatalyst as well as reduces surface friction, so that the silvercatalyst can glide out of a solidification zone and a dimension-stableammonium carbonate and a PTFE-binder takes up a roller pressure. 10.Method as per claim 7, comprising using in the first calender step, ahomogeneous catalyst band of thickness between 0.2–0.5 mm.
 11. Method asper claim 7, comprising adjusting a roller gap to 350 μm and setting aroller feed to approx. 2 meters per minute.
 12. Method as per claim 7,comprising using as electrical conducting material, a silver-coatednickel wire net with a string thickness of 0.15 mm and mesh width of0.45 mm with an approx. 10 μm thick silver precipitate.